In the past decade, contactless technology has been used extensively in card payment industry, such as debit cards, credit cards and charge cards. More recently, contactless technology has been extended to mobile phones that emulate contactless cards, enabling users to replace multiple cards with a single device. Accordingly, near field communication (NFC) technology has been introduced to support this application. Mobile phones and SIM cards that can store and run various software applications make a powerful platform to be utilized together with NFC. NFC is in fact a branch of radio frequency identification (RFID) operates at high frequency of 13.56 MHz and offers data transmission rate from 106 to 424 kbps within a short distance of few centimeters (typically up to 10 cm). The standards and protocols of NFC are outlined in ISO/IEC 14443, 18092, 15693, JIS X 6319-4, etc.
An NFC device can be an NFC initiator (or reader) as well as an NFC target (or tag), and may operate in different modes, namely, read/write mode, peer-to-peer mode, and card emulation mode. An NFC target is only activated when it is within the response range of an NFC initiator. NFC technology involves an inductive coupling of the initiator's antenna and the target's antenna, measured by a coupling factor (K) (value between 0 and 1). This coupling factor depends essentially on the geometric parameters of the antennas and the distance between them. In passive load modulation (PLM), switching of a load connected to the target's antenna is performed, in which a default load corresponds to the unmodulated state and a switched load corresponds to the modulated state. When the initiator's and target's antennas are inductively coupled, the initiator can detect these load variations and decode them to extract the information.
The difference between the modulated and unmodulated voltages sensed by the initiator is the load modulation amplitude. Generally, the higher of the coupling factor, the greater will be the load modulation amplitude. If the amplitude falls below a certain minimum value, the initiator may not able to reliably sense the signal modulations. For contactless payment cards (e.g. credit card of ID-1 format), PLM can reliably produce sufficient load modulation amplitude since a relative large antenna can be embedded in the card. However, this is not the case for NFC-enabled mobile phones as the cramped form factor of mobile phones may provide space for a very tiny antenna only (thus weak coupling factor). Furthermore, the abundant of metals, circuits and radio frequency (RF) signals of mobile phones may further impair the ability of a mobile phone to couple with the initiator's antenna.
In view of the above design constraints, to improve the performance of NFC mobile phones, active load modulation (ALM) technique has been introduced. Unlike PLM which utilises power from the RF field generated by the initiator device, ALM makes use of the mobile phone's battery power supply to actively transmit the modulated signal. In ALM, a modulated signal synchronous with the initiator's carrier signal is transmitted during the modulated state, and turned off during the unmodulated state. The main benefit of ALM technique is that the same load modulation amplitude can be achieved as PLM technique, but with a much lower coupling factor. Thus, the initiator device is unable to differentiate whether PLM or ALM technique is used to transmit the data and this resulting in an identical consumer experience. Accordingly, ALM permits the use of an antenna that is much smaller than the antenna of a typical PLM system, e.g. 80 to 90% size reduction.
To extend the load modulation range with smaller antenna size, ALM technique generates a signal with the same spectral characteristics as a load modulation signal and to actively transmit this signal to the initiator. More specifically, instead of using direct load modulation, the target uses a subcarrier frequency (fs) to modulate data. Various modulation schemes, such as amplitude shift keying (ASK), on/off keying (OOK), and binary phase shift keying (BPSK), and data encoding techniques, such as Manchester encoding, NRZ-L encoding, and Modified Miller encoding, may be used. When the load resistor of the target is switched on and off at a high frequency (fs), two modulation sidebands are created at a distance of ±fs from the carrier frequency (13.56 MHz) of the initiator. The subcarrier frequency, fs=13.56 MHz/16=847.5 KHz (for bit rate 106 kbps), and the upper sideband is located at 14.4075 MHz and the lower sideband is located at 12.7125 MHz. The data to be transmitted is contained in the modulation sidebands. Thus, to transmit data from target to initiator, the target needs to generate two subcarrier signals, each of which has sidebands containing the data to be transmitted to the initiator. The amplitude of such modulated signal may be amplified by a power amplifier and then radiated to the initiator by the target's antenna.
To ensure proper ALM transmission, besides the required load modulation amplitude levels, it is also requires that the transmitted modulated signal is synchronous (in phase) with the initiator's carrier signal. But phase synchronization is hard to achieve at all time, especially during ALM transmission as the carrier signal transmitted by the initiator is obscured by the active transmitted signal from the target. Thus, during the time when the target is actively transmits its data, the initiator's carrier signal is not directly observable. As such, the phase/frequency of the initiator's carrier signal may not be recovered faithfully by the target. This may result in a not synchronous target response and thus a phase drifting signal received by the initiator. Phase synchronization becomes even harder for asynchronous transmission as the target uses a local oscillator to generate a local reference clock which is independent from the initiator's oscillator.
One possible solution for phase synchronization is proposed in the U.S. patent application Ser. No. 13/482,930. In this method, the antenna of an NFC device is split into two loops for transmit (Tx) and receive (Rx), with a common ground. Ideally, the coupling between the Rx and Tx loops is zero, allowing the NFC device in target mode to receive and synchronize with the carrier's phase of the NFC device working in R/W mode even during transmission. However, study shows that a zero coupling between the two loops can hardly be ensured in practical implementation. Furthermore, due to its complexity and high fabrication cost, such an antenna with two loops is not favourable.
In light of the above, those skilled in the art are striving to improve the phase synchronization technique in order to prepare a synchronous carrier signal for ALM transmission.